Performing Ultrasound (US) of the lungs was initially thought to be impossible given air is the enemy of US. This changed in 1995 when Lichtenstein published an approach to imaging pneumothorax with US. Since then, analyzing artifacts produced by the normal and pathologic lung has become central to diagnostic lung ultrasound. 

Indications

Established/great evidence

• The critically ill patient presenting with dyspnea
• Pneumothorax (PTX)
• Pleural effusions
• Acute Decompensated Heart Failure (ADHF)

Developing/emerging literature

• ARDS vs ADHF
• Consolidation/PNA
• Airway management
• Neonatal 

Specific Lung US Artifacts

Lung sliding sign and the pleural line

• Should be the anchoring point of all lung US exams to assure that what you are imaging is truly the lung
• The initial reference point for both normal and pathologic findings
• Produced secondary to sliding of the visceral and parietal pleura to and fro during ventilation

A-lines

• An artifact of normal lung, best visualized in hyperinflated lungs (COPD) given more air, or extrapleural free air (PTX)
• Best produced by free air, but well produced by aerated lungs, generally absent in interstitial syndromes, consolidations or effusions
• Horizontal hyperechoic lines projecting at regular intervals throughout the depth of the screen
• These lines represent the pleural line
  • When US waves hit the air in alveoli or free air in a PTX just beyond the soft tissue and the pleural line, almost all of it is reflected back towards the probe
  • This causes reverberation between the chest wall and the pleural line creating the artifact seen as a horizontal hyperechoic line
  • The distance between A-lines is a regular interval representing the distance between the probe surface and the pleural line

B-lines (aka comet tails)

• Discrete, laser-like, vertical hyperechoic reverberation artifacts that arise from the pleural line, extend to the bottom of the screen without fading and move synchronously with lung sliding and ventilation
• Erase A-lines
• Occur in interstitial states where there is extra-alveolar fluid collection (ADHF)
• Some ( less than 3) B-lines can be present in normal, non fluid-overloaded lungs
• Produced by reverberation of the sound waves in between air filled alveoli and fluid filled interstitium
  • This is reflect back to the probe and the machine interprets this as infinite repetitions of tiny surface areas and produces the B-line as the 2D representation

Probe Selection & Positioning

• No consensus on the best probe to use
• High frequency linear probe good for PTX evaluation
• Lower frequency curvilinear or phased array necessary for deeper evaluation of B-lines, consolidations, pleural effusions
• Curvilinear or phased array probes better for the evaluation of dyspnea in acute patients given the use of one probe increases efficiency of exam
• Less image processing is better given lung US makes use of artifacts
  • Turn off filters, high frame per second
  • Image processing generally decreases unwanted artifacts which is desirable in most other US exams
• Patients should be placed supine for the majority of the lung exam
• Extrapleural air will move anteriorly
• Extrapleural fluid will move posteriorly

From Volpicelli, G et al. (2012). International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Medicine 2012 38: 577-591

Approach Based On Indication

Pneumothorax (PTX)

• Patient supine, start with transducer oriented vertically on anterior chest wall
• Locate 2 ribs so that the pleural line can be identified correctly
• Once you find lung sliding indicating the pleural line rotate the probe horizontally and then slide within the rib space to assure there is no area lacking lung sliding
  • You can confirm lung sliding with M-Mode and you should visualize the “seashore sign” as above
  • The “sand” of the “seashore sign” is produced by haphazard reflection of the sound waves in multiple different directions off of the spherical alveoli
• Air should accumulate most anteriorly in the supine patient so it should be adequate to image just the anterior chest
• If an area of no lung sliding is found, you can use M-mode to confirm which will not show the typical “seashore sign” but will show the “stratosphere sign”
• Pathognomonic for a PTX is the “lung point sign” which shows the point at which the lung loses contact with the chest wall
• Lichtenstein’s 1995 article found sensitivity of US of the anterior chest wall to be 95.3% sensitive and 91.1% specific for PTX with a NPV of 100%
• Since then multiple studies have shown superiority of POCUS in ruling out PTX

Pleural effusions

• Patient in a supine position
• Gravity dependent, should flow to the most dependent regions of the pleural cavity
• Image the postero-inferior aspect of the thorax as this is where fluid will accumulate
• The optimal place to visualize a non-loculated pleural effusion is the posterior axillary line just superior to the diaphragm
• M-mode can help differentiate small effusions from pleural thickening if you visualize the sinusoid-sign (swinging of the lung to and fro in the fluid)
• More sensitive than CXR and as sensitive as CT

Pulmonary Edema

• B-lines in multiple fields is suggestive of the diagnosis
• It has been proposed to separate each hemithorax into 4 segments
  • If B-lines present in >2 segments the exam is positive for interstitial fluid
  • A positive region is defined as: the presence of three or more simultaneous B-lines in a single intercostal view
• Some suggest using the 28 point lung exam (scanning at 4 different vertical points on the parasternal, mid-clavicular anterior axillary and mid axillary lines
  • Increases sensitivity however impractical given time considerations
• In the critically-ill patient it may be sufficient to just examine two anterior chest interspaces given time constraint
• In the acutely dyspneic patient lung US identifying the presence or absence of B-lines can be key in differentiating between the diagnosis of COPD exacerbation and ADHF

Pearls and Pitfalls

• Failing to properly identify the pleural line
  • Soft tissues and muscles are frequently mistaken for the pleural line if the patient is hyperventilating
  • In a tachypneic trauma patient, this can easily be called a false negative for PTX
• Subcutaneous emphysema
• Not suspecting a pulmonary embolus (PE) with a normal lung exam
  • Normal lungs are typical of patients with PEs
• Not thinking about performing lung US or thinking it will take too long

References

• Ma, O.J., Mateer, J.R., Reardon, R.F., & Joing, S.A. (2014). Ma & Mateer’s Emergency Ultrasound. 3rd Edition. New York, NY: McGraw Hill.
• Lichtenstein, D., MD, & Mezière, G. A., MD. (2008). Relevance of Lung Ultrasound in the Diagnosis of Acute Respiratory Failure: The BLUE Protocol. Chest 2008 134: 117-125
• Volpicelli, G et al. (2012). International evidence-based recommendations for point-of-care lung ultrasound. Intensive Care Medicine 2012 38: 577-591